In recent years, white light sources including light emitting diodes (LEDs) have been attracted attention in terms of energy saving measures and a reduction in the amount of emitted carbon dioxide. Compared with conventional incandescent bulbs including tungsten filaments, LEDs have longer service life, and enable energy saving. As disclosed in Japanese Patent Laid-Open No. 10-242513 (Patent Document 1), in conventional white LEDs, YAG phosphors are excited using blue LEDs each having a light emission peak wavelength in a range of 400 to 530 nm, and the blue light emitted from the LEDs and the yellow light emitted from the YAG phosphors are mixed with each other, whereby a white light is achieved.
White light sources that use LEDs have been widely used as backlights of traffic signal lights and liquid crystal displays as well as general lighting equipment such as room lights. In the light emission spectra of conventional white light sources including blue LEDs, the peak height of the blue light emitted from the blue LEDs is as large as at least 1.5 times the peak height of the yellow light emitted from phosphors, and hence influences of the blue light tends to be strong. FIG. 1 shows a light emission spectrum of a white light source described in Patent Document 1.
Under the circumstances, with the popularization of the white light sources including LEDs, diverse demands are being made with respect to white light sources. With the white light sources in which blue light emitting diodes and YAG phosphors are combined as described in Patent Document 1, there have been arisen the problems that the light may appear yellowish depending on the viewing direction and that irregular colors of blue or yellow may arise.
In order to solve such problems, for example, the pamphlet of International Publication No. WO 2007/037120 (Patent Document 2) discloses a white light source in which an ultraviolet light emitting diode and phosphors are combined. A white light source having high color rendering properties is realized by combining the ultraviolet light emitting diode and three types of phosphor, namely, a blue phosphor, a green phosphor, and a red phosphor, thereby to realize a white light source having a high color rendering property. FIG. 2 illustrates a light emission spectrum of the white light source described in Patent Document 2.
In the white light source described in Patent Document 2, in order to achieve high color rendering properties, a light emission peak in a red region is set so as to be high as shown in FIG. 2. When illumination is performed by using the white light source having such a light emission spectrum, the color tones of an object of irradiation (object illuminated by the light) can be seen clearly. On the other hand, when clothes or the like are illuminated, in some cases a significant difference may be perceived between the tones when the clothes or the like are irradiated with a white light source having high color rendering properties and the tones that are perceived under irradiation with sunlight. That is, with respect to the case of irradiation with a white light source having high color rendering properties and the case of irradiation with sunlight, the problem arises that a significant difference in color tone is perceived even though the object of irradiation is the same.